The invention relates generally to ground fault circuit interrupters and, more particularly, to ground fault circuit interrupters providing protection against grounded neutral conductors.
Conventional electrical circuits such as fuses and circuit breakers protect circuit conductors from thermal damage due to severe overload currents, thereby greatly reducing the danger of fire and explosion. However, such conventional circuit interrupters do not eliminate the danger of electrical shock to a person accidentally coming into simultaneous contact with a live conductor and an object at ground potential. The resulting current flow through the person, while only a fraction of an ampere, can cause serious injury or death.
Ground fault circuit interrupters (hereinafter referred to as GFCI's) combine the capabilities of conventional circuit breakers with sensitive means for detecting current flow between line conductors and ground at current levels much below the overload current levels required to trip conventional circuit breakers. Upon detection of such a ground fault current the contacts of the GFCI are opened to deenergize the circuit.
A differential current transformer is normally used to sense these ground fault currents, the transformer having as its primary windings the conductors of the distribution system being protected. During normal conditions, all current flowing in one direction through one of the conductors will return in the opposite direction on another of the conductors, this producing a net current flow of zero through the transformer. However, if a fault (that is, a leakage path) is established between one of the conductors and ground, return current will bypass the transformer and flow through the ground back to the grounded side of the source supplying the circuit. Thus, more current will be flowing in one direction through the transformer than through the other, producing a current imbalance.
A sensing winding detects this imbalance and provides an output signal used in various ways for the common purpose of tripping a circuit breaker mechanism when the sensed signal is of sufficient magnitude. One method of utilizing the signal of the sensing winding to produce a trip indication is described in U.S. Pat. No. 3,852,642 issued Dec. 3, 1974 to the present inventor and others and assigned to the assignee of the present invention. The device disclosed therein responds primarily in accordance with the peak value of the sensing winding signal.
Such circuits resulted in generally satisfactory operation. However, performance standards for GFCI's as established by Underwriter's Laboratories have been increased to specify a trip level of 5 ma..+-.20% for all ambient and load conditions. The trip levels of some prior art GFCI's were dependent upon normal load currents to the extent that the tightened specifications resulted in an increase in unnecessary trip indications, often referred to as "nuisance tripping". It is believed that this effect is caused primarily by a false output from the differential current transformer at a frequency equal to twice the line voltage frequency; that is, 120 Hz on a 60 Hz system. The false 120 Hz output, when added to the actual 60 Hz output, results in a composite output current the peak value of which is a function of load current. This false output is believed to be caused by stray magnetic fields existing in the vicinity of the current transformer caused by difficult to control variables such as the exact location of current carrying conductors near the transformer within the circuit breaker, input offset voltage for the sense amplifier, residual flux in the core, and others. A direct solution to this problem would include shielding the transformer and using more symmetrical lead routing. Unfortunately, the size and space restrictions within the housings of GFCI's do not always permit such shielding, and the conductor locations are dictated by manufacturing considerations.
A method for alleviating the problems caused by 120 Hz false current is to make the trip indication dependent upon an integral of the current transformer output. As example of this approach is the device described in U.S. Pat. No. 3,953,767 issued Apr. 27, 1976 to Ahmed. This device sums the sensed signal over a period of at least a fully cycle of line voltage primarily for the purpose of distinguishing between resistive ground faults and capacitive ground faults. This has the incidental benefit of reducing any 120 Hz signal which may be present, since over each half cycle of power line frequency the integral of a 120 Hz will be zero.
There are, however, other requirements for a commercially practical GFCI. Underwriter's Laboratories specifies that a GFCI must also trip upon occurrence of a low-impedance leakage path from the neutral conductor to ground. Such a path on the load side of the differential current transformer does not in itself produce a shock hazard; however, the occurrence of a grounded neutral at the same time as a ground fault on a line conductor will cause the GFCI to be less sensitive in detecting ground fault current from the line conductor.
Various means have been successfully employed to detect a grounded neutral conductor, including the device described in U.S. Pat. No. 3,959,693 issued May 25, 1976 to Coley and Misencik and assigned to the assignee of the present invention. The device disclosed therein employs an additional transformer having a primary winding connected between the line and neutral conductors, with the neutral conductor serving as a secondary winding. The core of this neutral transformer is designed to saturate early in each half cycle of the power line frequency. The transformer thus induces a voltage pulse on the neutral conductor on each half cycle, producing a current flow on the neutral conductor if there is a path from it to ground near the load. Since this current returns through the ground, a current imbalance will result which will be detected in a manner similar to a ground fault by the differential current transformer to produce an output from the sense winding.
Another means for detecting a grounded neutral conductor employs a pulse generator inducing high frequency voltage pulses upon the neutral conductor. This method is described in U.S. Pat. No. 3,611,035 to Douglas.
Both of the foregoing techniques are generally effective to detect leakage paths from neutral to ground. However, both produce a ground current having a fairly large peak value but a very limited average value over one cycle of the power line frequency. Thus, these techniques as previously utilized are not directly applicable to apparatus employing an integration technique, such as the device described in the aforementioned U.S. Pat. No. 3,953,767, since the pulses provided by the grounded neutral detection system would be cancelled by integration.
A ground fault detection system providing grounded neutral protection and utilizing an integrator is described in U.S. Pat. No. 3,963,963 issued June 15, 1976 to Schade. This system employs two synchronous switches, one operating at line frequency and the other operating at the frequency of the oscillator generating the neutral conductor pulses, to activate the integrator only at such time as the positive neutral pulses are occurring. Various problems still remain, however, such as the dependence of trip level on load current.
It would be desirable to provide a GFCI having grounded neutral protection which meets current Underwriter's Laboratories requirements as established in Bulletin 943 which call for a 5 ma. trip level .+-.20%. In addition, the device must be sufficiently compact to fit in existing circuit interrupter housings and must minimize the volume and number of parts required. It would be especially desirable to provide a GFCI which can be adapted for operation using an integrated circuit.